The present invention relates generally to MR imaging and, more particularly, to a method and apparatus to reduce ringing artifacts in multi-echo acquisition using a non-discrete flip angle train. The present invention is particularly applicable with fast spin echo (FSE) imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
As is well-known, reducing the duration of an MR scan has a number of advantages. For example, as scan time is reduced, patient throughput increases. An increase in patient throughput allows more subjects to be imaged in a given period of time or support more comprehensive scans without a time penalty. Additionally, it is generally well-known that some subjects, and in particular, children, the elderly, and those that are claustrophobic, are prone to movement during the data acquisition process. This movement can introduce motion artifacts in the final reconstructed image thereby jeopardizing the diagnostic value of the final image. Accordingly, by reducing the duration of the scan it is possible to reduce the likelihood of subject motion induced artifacts in the reconstructed image.
FSE is an imaging technique that has been developed and widely known to reduce scan time. FSE is widely used for spin—spin relaxation weighted imaging, proton density imaging, and spin-lattice relaxation weighted imaging in relatively short periods of time. Moreover, FSE imaging may be implemented for neural imaging, body imaging, and extremity imaging.
FSE imaging utilizes a multi-echo, spin-echo pulse sequence where different parts of k-space are acquired by different spin echoes. For example, a four echo spin-echo sequence may be applied such that k-space is segmented into four sections. As such, the first echo may be used to fill a center of k-space, the second echo for k-space adjacent to the center, and so forth, with the data from the last spin-echo used to fill the outermost regions of k-space. Since four echoes rather than one are used to fill k-space, scan time, in this example, may be reduced four-fold.
In FSE imaging with long echo trains, the observed echo amplitudes typically exhibit an amplitude decay based on tissue T1 and T2 values. This decay can result in undesirable ringing artifacts, particularly in long echo train images. Additionally, with high field acquisitions, e.g. 3T and above, maintaining RF power deposition (SAR) in the imaging bore during the acquisition process is critical. This is particularly critical for multi-echo acquisitions such as FSE. Several strategies have been developed to address both signal amplitude decay and RF power deposition during multi-echo acquisitions.
One known strategy utilizes a variable flip angle train to improve image quality and maintain RF deposition within prescribed limits. However, current state of the art techniques using this strategy provide discrete flip angle trains that are not flexible to take into account prescription parameters such as echo train length, echo spacing times. As such, these techniques yield flip angle trains that are not matched to the parameters of the prescription. Also, these techniques generally fail to take into account changes in tissue type when determining the respective flip angles of the data acquisition pulses of the multi-echo acquisition.
It would therefore be desirable to have a method capable of automatically determining the flip angles for each data acquisition pulse of a multi-echo acquisition that is based on tissue type as well as scan prescription parameters on a per-scan basis.